The general behavior will usually fall into one of the following categories:
1. It gets stuck and repeats a fraction of a second (1 rotation).
2. It gets stuck, jumps back, and repeats a few seconds.
3. It starts having repetitive noise at the disc rotation rate - about
200-500 rpm (3-8 Hz).
4. It starts skipping continously or every few seconds either forward
or backward.
Assuming your CD is clean and undamaged (check with different CDs),
then this sounds like a mechanical problem - proabably dirt in the
optical pickup worm screw or lack of or dried up lubrication.
It could also be a worn spindle bearing or an electronic adjustment.
If problems are most severe at the start of a disc, then spindle motor
problems or PLL adjustments are likely possibilities.
If problems are most severe near the end of a disc, spindle bearing,
track lubrication, and PLL adjustments are likely possibilities.
The next few sections deal with these types of problems in detail.

This means jumping forward or backward by a fraction of a second. It may
occur occasionally or may appear as though the pickup is bouncing across
the disc.
Common causes include dirty lens, dirty or damaged disc, need to adjust fine
tracking offset/gain or tracking balance, weak laser or other defective part
in the optical pickup.
First, inspect the disc for badly scratched or smudged areas and other defects
or try another one. Clean the lens. See the chapters: "Servo Systems and CD Player Adjustments" and "Testing of Optical Pickup Assemblies".

This means repeating the same track or a small number of tracks (meaning
disc rotations, in this case). The effect is somewhat like a 'broken
record' with an LP but at a much faster rate - 3 to 8 repeats per second
when repeating only a single track.
The most common underlying cause is a damaged or dirty disc. However, if
the tracking (and sometimes focus as well) servos are not properly adjusted,
the CD player may exhibit excessive sensitivity to disc problems.
If the focus or tracking gais is set too high or the offsets/balance are not
centered, slight disc imperfections, scratches, or dirt may result in this
set of symptoms.
See the chapter: "Servo Systems and CD Player Adjustments".

Usually, several seconds of music will play without any trouble and then
there will be a skip forward or backwards by a few seconds or longer. In
the latter case, the net effect may be to constantly repeat a section of
the CD. Make sure you do not have any repeat modes enabled!
Common causes include a dirty lens; dirt, foreign materials, or lack of
lubrication in pickup drive; defective disc (surface defects, dirt, or
fingerprints); mechanical damage causing mechanism to bind.
* First, inspect the disc for badly scratched or smudged areas and other defects
or try another one. Clean the lens.
* A mechanical fault is quite likely. These symptoms generally indicate that
the coarse tracking servo is unable to properly move the pickup easily as
it should - it is getting stuck and then either jumping back once the error
is too great or breaking free and moving forward in spurts.
* Eliminate the possibility of mechanical problems - see the section:
"Testing the sled for mechanical problems".

Common causes: transportation lock engaged, gummed up lubrication on pickup
tracks or worm gear, other mechanical problems like an obstruction or errant
wire getting in the way. A flex cable with a hairline crack in one or more
conductors might also cause this symptom.
* Make sure the transportation lock, if any, is disengaged.
* Carefully inspect the sled gears and tracks for dirt and gummed up lubrication.
If the player has been in commercial service always playing the same CD or
set of CDs and now you are attempting to one that is someone longer, this may
happen as the end of the track is unused and dirt collects at the boundary.
* Check for mechanical damage and obstructions like wires or cables.
* If you are attempting to play a CD which is longer than 74 minutes, the player
may not be capable of accessing the last part of the CD. It might either abort
or get stuck and keep repeating a fraction of a second or several seconds. See
the section: "Problems with extended length discs"
* If play deteriorates gradually as the pickup moves toward the outer edge of
the disc, the CLV servo may need adjustment or the spindle motor may be
defective.

This means that one part of the disc (start or end) plays properly (or at least
with less problems) than another. For example, the disc may play flawlessly
until approximately the 30 minute point and then develop noise, skipping, or
other similar problems.
Common causes: defective disc, faulty spindle motor, misalignment of spindle
table and sled track, need for CLV adjustment.
* Try some other discs to eliminate a defective disc as a possibility.
* If the problem is most severe at the start if the disc, the spindle motor may
have trouble reaching the required 500 rpm rotation rate consistently. See
the chapter: "Motors and Spindles".
* The spindle table and track on which the sled moves may be misaligned. This
is especially likely if the player was dropped or otherwise abused. See the
section: "Testing the sled for mechanical problems".
* An adjustment of the servo that controls the Constant Linear Velocity (CLV)
drive to the spindle motor may be needed. See the chapter: "Servo Systems and CD Player Adjustments".

Common causes: dirty lens, bent spindle, excessive runout (wear) of spindle
bearing, loose spindle, foreign material on disc table, disc not firmly
clamped, warped disc, need to adjust focus or fine tracking offset/gain,
weak laser.
* First, inspect the disc for badly scratched or smudged areas and other defects
or try another one.
* Clean the lens.
* Check for a loose spindle (sometimes there is a set screw that needs to be
tightened or some adhesive may have broken free.
* Make sure there is no dirt or other foreign matter on the spindle table that
could cause the disc to seat improperly.
* Observe the disc as it spins. Is the edge moving up and down by more than
a total of about 1 mm? If so, the disc may be excessively warped, or possibly
the spindle bearing is worn resulting is unacceptable runout, or (unlikely
unless the player was thrown off a cliff) the spindle is bent. The focus and
fine tracking servos may be incapable of correcting such a large wobble.
This could also be due to a disc clamper that is not working properly - the
drawer closing mechanism may not be quite completing its cycle or possibly
the magnet may have weakened. Gently press down on the rotating part of
the clamper while playing - if this reduces or clears up the playback and/or
if you can feel the disc seat better, then this is a possibility.
* If the problem developed gradually and has been getting worse, than a worn
spindle bearing is a distinct possibility. Adjustment of focus and fine
tracking offset (or possibly gain but usually less critical) may help.
* Alternatively, focus or fine tracking offset (or possibly gain but usually
less critical) may simply have drifted a bit and adjustment is all that is
needed.
* A weak laser may also result in these symptoms but do not attempt to
adjust laser power until other possibilities have been investigated fully.

This may mean that some discs play properly while others have problems with
skipping, sticking, repeating, or noise in the audio. Unfortunately, many
players, particularly portables and CDROM drives, do not have adjustments
that are labeled. (For some portables, you may need the special test discs
described in the section: "Useful ways to mangle CDs".
Since one possible cause of these types of problems (after the lens and
mechanics have been ruled out) are servo adjustments. See the chapter:
"Servo Systems and CD Player Adjustments".
Then, I would mark to precise positions of all the controls. While playing a
disc that works but has minor skipping, noise, or similar problems, carefully
try adjusting each one on either side of its current position to see if that
will help. Then if this helps, change to progressively more problematic discs
to see if you can home in on the optimal settings. By observing the behavior
as you tweak each control, it may be possible to determine their functions.

Binding or obstructions would be indicated by any long distance skipping,
jumping, repeating, or failure to seek or search past some location (time)
on multiple discs. Defective or erratic limit switches may result in
jamming or overrun at start or end of disc or unreliable reset during
startup.
Check for free movement of the optical pickup sled on its tracks or bearings.
Manually rotate the appropriate motor or gear or in the case of a voice
coil (linear or rotary) positioner, gently move the pickup back and forth
throughout its range. There should be no sticky positions or places where
movement is noticeably more difficult. If there are, inspect for mechanical
problems like broken or damaged gear teeth, dirt or other material that
should not be there, and gummed up lubrication - or that you didn't forget
to release the transportation locking device! Damaged parts will need to be
replaced (or repaired - sometimes a fine file, Xacto knife, or dental pick will
work wonders but don't count on it). Otherwise, cleaning and lubrication may
be all that is needed. Remove the dirt and the old gummed up lubricants and
lubricate the tracks and/or gears using the proper oil or grease. (See the
sections: "Lubrication of CD players"
Inspect the alignment of the track with respect to the spindle motor. If
the spindle motor shaft is not at an exact right angle to the sled movement,
tracking may be affected on certain portions of the disc. One best way to
this is to clamp a disc onto the spindle table and then manually move the
sled from end-to-end measuring the distance between the pickup and disc at
both extremes. It should be equal. A variation of more than a fraction of
a mm can cause tracking problems.
If these tests come up negative, check out the pickup (sled) motor for defects
such as a shorted or open winding, dead spot, partially shorted commutator, or
dry or worn bearings. See the section: "Testing of motors".
As a double check, disconnect the motor from its driving circuit (extremely
important!). Use a 3 V battery in series with a 25 to 50 ohm variable resistor
or a variable low voltage (e.g., 0 to 5 V) DC power supply to drive the motor.
Start at the highest resistance or lowest voltage and adjust it until the sled
just starts moving. Run it from end-to-end in both direction. The sled speed
should be fairly uniform with no sticking or binding. There should not be any
excessive noise or grinding sounds. If this is not the case, there are still
problems with motor or sled mechanism.
Another check would be to substitute a 15 ohm 1 W resistor for the motor and
see if a few volts appears across this when the player should be resetting
since this usually involves moving the sled to the inner limit. If there is
none, the driver may be blown or lack power, or the controller is not providing
the proper commands.
In addition, check the proper functioning of any limit switches that are
present on the optical deck. There will almost always be one for the inner
(reset - startup) track and there may be one for the outer track (end-of-disk)
as well. Run the pickup manually or using the battery (see above) to both
ends without forcing and check for reliable operation of the switch contacts.

Silly me, what other kinds of problems are we talking about? OK, I should
have said: audio signal chain problems. The player appears to be working fine
(the disc is spinning; the time is incrementing correctly; seek, search, and
play operations behave normally) but there is either total silence, noise, or
distortion, on one or both channels.
However, also see the section: "Types of skipping problems" and those that
follow since these sorts of audio symptoms may be mistaken for those caused by
problems with servo alignment, the optical pickup, or front-end electronics.
There is a distinct boundary between the digital section where audio
information is encoded digitally and the analog domain where it is an
electrical waveform.

Problems in the digital domain will usually be obvious to the point of being
gross - extreme noise, noise correlated with the signal level, extreme
distortion, tones or frequencies that with no stretch of the imagination
were present in the original music, etc.
Characteristics will be distinctly different than the kind of noise or audio
distortion we are accustomed to in stereo equipment. Small errors in the
digital reconstruction can result in totally gross changes in the audio output.
For example, a single bit error if in the MSB can totally corrupt the resulting
waveform. Simple errors can result in sound frequencies not present in the
original. Fortunately, these sorts of errors are relatively rare as most of
the circuitry is inside of very reliable LSI chipsets.
However, if the CD is recognized and appears to behave normally except that
there is absolutely no audio output, there can be problems in the audio
decoding LSI chips. Other than hoping for an obvious bad connection, this
is way beyond the scope of anything you can hope to repair without the service
manual, test equipment, and a miracle.

Problems in the analog sections - D/A(s), sample-and-hold(s), post analog
filters, and muting relays - produce effects that are more familiar: noise,
decrease in signal strength, and distortion. Except for parts of the D/A
which may be shared, there will be identical left and right channels to
compare if an audio problem develops.
If only one channel is affected, then the problem most likely has nothing to do
with tracking, the laser, or the mechanism. Coming off of the disc, the left
and right channels are interleaved on a sample (16 bit word) basis so any disc
or pickup problem would equally impact both L and R.
You are left with the D/A and sample-and-hold or D/As or the final analog
filter and muting circuitry. Many CD players multiplex the D/A between L
and R, so in these, even the D/A is ruled out since most of its circuitry
is common.
Swapping components one at a time between the identical left and right
channels is also a valid diagnostic technique.
* With a single D/A, there will be sample-and-hold circuits for each channel as
well.
* Players without digital filters (or oversampling) have fairly complex analog
filters after the D/A. A bad or noisy component could conceivably be your
problem. Even players with fancy oversampling have some kind of a final
analog (antialiasing) filter. On an older player, there is probably a lot
of discrete circuitry in the audio section.
* If you can get to the components in the analog filter (some are potted), then
with a test CD which has a 'silence track' and a scope or signal tracer, you
should be able to find out where the noise is being introduced. If it has
separate D/As, then one of these would also be suspect.
* There may be separate power supply outputs for the audio section (this will
be more likely with fancy expensive players). In this case, a failure of
one of these may result in either distorted audio or no sound at all.
The following will mostly result in static type noise, hum, or erratic audio
(sound not coming on or partial or total dropout for one or both channels):
* Don't overlook the simple problems of dirty contacts on the RCA jacks or bad
connections where they are soldered to the main circuit board. Test by
jiggling the cable connectors and/or prodding the circuit board near the
RCA jacks. The cable may be bad (from flexing) as well - try another one.
Check the connections and controls on your amplifier and other audio
components as well! Any bad connection in the audio path can lead to
these symptoms. Clean, repair, or replace as appropriate. Perhaps your
poor, helpless CD player isn't even at fault!
* Dirty muting relay contacts may result in intermittent or noisy output.
If tapping the relay affects the symptoms, this is likely the problem.
To test, remove the relay and bypass the suspect contacts with jumper wires.
CAUTION: Turn your amplifier's volume control down when you start to play
a disc - there may be unusual loud noises during startup that are now not
blocked by the muting circuitry.
If CDs now play without any audio problems, a bad relay is confirmed.
It may be possible to snap off the cover(s) and renew them with contact
cleaner and a burnishing tool or a strip of paper. Otherwise, replacement
will be required.

My first thought would be to say "how can the electronics know about the
voice separately?" Well, the answer is "it cannot". However, due to the
way vocals are often recorded, this behavior is possible, if unlikely.
What must happen is for the audio output to be the difference between the
left and right channels mostly cancelling the centered vocal track but not
having as much effect on the audibility of the instruments.
It is possible for this to happen as a result of a bad ground connection or
an electronic fault in the analog circuitry following the D/A stage but it
is quite unlikely to be due to a problem in the optical pickup or digital
decoding - though not out of the question.
What is suggested below can happen by accident should the shield connection
to the headphone or line out jack or cable become disconnected.
(From: Frank Fendley (frank.fendley@datacom.iglou.com)).
Actually, it is possible. Modify a headphone so that the two ground
conductors are still connected together and to each earpiece, but no longer
connected to the sleeve of the headphone jack. The two "hots" remain
connected to tip and ring on the jack. Plug it in to a portable CD player
and listen to a pop or country CD with (preferably) a solo vocalist.
The vocals will almost disappear, while the instruments will still be
quite audible (although now in mono).
Normally, the lead vocals are 'centered' in the stereo imaging and are
in phase on each channel. The instruments are panned more or less left
or right. When you rewire the headphones, you effectively place the
two transducers in series, and they are now wired out of phase with
each other (the two "-" terminals are connected together on what used
to be the ground lead, and the two "+" terminals are connected to their
respective signal outputs from the channels - effectively since they
are now in series, they are wired out of phase). Any signal which
is "identical" and "in phase" on both left and right channels tends
to cancel - the vocals. Signal which is not identical on each channel
appears as the difference between the two sides - the instruments.
Some recordings are not made this way and this will have little or no
effect - you may have to try a few CD's to experience the 'phenomena'.

The specifications for the length of an audio CD is just over 74 minutes. I
have seen them as long as 78 or 80 minutes which means that some of the basic
CD specifications have been compromised - either the track pitch has been
reduced or the track extends closer to the outside edge of the disc - or both.
If the track pitch has been reduced, there could be tracking or audible noise
problems throughout the disc. If the track extends closer to the outer edge,
there could be problems near the end of the discs. The player may not these
discs at all. Any of the following symptoms are possible:
* No problems. Your player is one of those that is perfectly happy playing
really long CDs. Most players will indeed be unaffected.
* The disc may be rejected resulting in the display showing 'disc' or 'error'
as though damaged, improperly inserted, or missing. In this case, the
CD player's microbrain simply thinks anything with a total playing time
exceeding 74 minutes and 15 seconds is invalid.
Unless you want to redesign the player, there is nothing you can do to play
these CDs. It might only require changing a single byte in the player's
firmware :-).
* There may be more of a tendency for skipping, sticking, or audible noise
(probably near the end though it could happen anywhere if the track pitch
has been reduced - including inability to read the disc's directory) since
the servos are operating slightly outside their normal range. The actual
likelihood of these types of problems are very slight, however.
It may be possible to adjust the servos as described in the chapter: "Servo Systems and CD Player Adjustments". As with any adjustments, there is some
risk of affecting performance for all discs - or totally messing things up.
Or, if problems only occur near the end of these discs, just don't play them
to the end!
* The sled on which the pickup is mounted ventures into new territory where
no sled has ventured before (at least on this player). Dirt, gummed up
grease, hair, and other garbage may have collected there resulting in the
sled drive mechanism getting stuck. You may hear whirring, buzzing, or
clicking as the motor attempts to move the immovable - or nothing at all.
Eventually, the player should probably shut down. In any case, kill power
or remove the batteries to prevent damage to the gears.
With luck, all you need to do is move the sled manually toward the spindle
by turning the proper gear (freeing it up first, if necessary). Then clean
and relube the track and gears. Hopefully, nothing is actually damaged
since locating a replacement part may prove to be a challenge.
* The sled simply overran the end of the rack and the drive gear no longer
is capable of returning it to more familiar territory. You may hear some
whirring, buzzing, or clicking as the gears attempt to mesh but do not
quite succeed.
Manually moving the sled so that the drive gear meshes with the rack - and
then turning it a bit to be sure - should restore operation but, of course,
you should not attempt to play these extended length discs to the very end
in the future.

CD-Rs (recordable CDs, usually gold on the label side and greenish on the
readout side) can be quite variable in quality. They are often produced
on a low cost writer of questionable design and calibration. It is quite
common for a CD-R disc to play/read fine on one drive and not be recognized
at all on another. There may not be any relation between cost of the CDROM
drive and its reliability with CD-Rs.

Consistently recording high quality CD-Rs is by no means as fool-proof as
reading typical CDs. Any problems affect the recording quality permanently.
* Media - there is significant variability in the quality and consistency of
CD-R blanks from different manufacturers. You may have to experiment with
multiple brands to determine those that work for you in your CD-R writer.
* Recorder - there may be significant variability in the performance of
various manufacturer's hardware. High price may not translate into high
quality especially considering the rapid changes in the industry.
* Writing speed - while it really should not matter whether you record at 1X
or 4X (or whatever your machine supports), this may not actually be the
case. If the servo systems are less stable at the higher rate, the quality
of the recorded information may suffer. Thus, writing at a slower rate may
be better - or may not matter. In any case, experimentation at different
writing speeds should determine if this is an issue.
* Media cleanliness - you only get one shot. A speck of dust or fingerprint -
which might just decrease the signal to noise ratio when reading a CD - can
degrade the writing laser beam resulting in malformed pits (oh no, not the
dreaded malformed pit disease!). Inspect each CD-R blank before inserting
it into the writer. Reject it if you see any visible surface damage or
manufacturing defects. Use clean, dry compressed air if necessary to blow
off any dust or fluff. Clean the surface as you would a CD to remove any
fingerprints or smudges.
* Equipment maintenance - keep the recorder clean - periodic inspection and
cleaning similar to that used for CD players may be needed if it is used
in a less than ideal environment - dust, smoke, and cooking vapors can
quickly coat the lens leading to lower quality recordings. Inspect, clean,
and replace (as necessary) the caddies (if used) as well since dirty or
damaged caddies can cause problems as well.
* Data under-runs - where the recorder does not have an internal buffer of
sufficient size (yeh, like 650 MB!), it expects to be fed at a high enough
rate to always have data available to send to the writing laser. Any
failure will likely result in incorrect data being written - and probably
a ruined disk. Some recorder software will simply abort. Even running
another application like a screen saver can result in uncertainties with
respect to data availability. When in doubt or where time is available, run
the recorder at a slower speed to reduce the required datarate.
* Mechanical shock - locate the recorder on a stable surface - not the top
of a printer or other equipment that may be subject to movement during the
duration of a recording session. Any vibration transmitted to the optical
deck may cause a momentary shift in the position of the lens and laser
beam reducing the recording quality. Bump it hard enough and the result
will be mistracking and a ruined disk.

Newer portable CD players often offer an extra cost option of an 'anti-skip'
capability - usually about 10 seconds of buffer memory. While there is
probably little you can do to repair an electronic or logical problem with
this memory, there are a couple of points to keep in mind which may lead
to the repair of problems like erratic anti-skip performance, noise, dropouts,
skipping, and other symptoms dealt with elsewhere in this document.
Anti-skip is actually implemented by reading ahead on the CD and storing
up to 10 seconds of digital audio in dynamic random access memory (DRAM).
This has a direct impact on optical deck performance and power requirements:
* To read ahead, the player must actually operate at a higher than normal
(1X) speed. Watching a player with this feature, it would appear to be
close to 2X. This puts a greater strain on motors and servo systems so
anything in the optics or servo alignment that is marginal - or even a
dirty lens - may result in problems which do not show up with the anti-skip
feature turned off.
* Power requirements are also greater with anti-skip on - the spindle motor
and servos need to work harder and the buffer DRAM may require greater
power when being accessed. Therefore, weak batteries or an inadequate
wall adapter may result in erratic operation. If possible, try fresh
batteries or a different adapter before warming up the oscilloscope.

The newest CDROM drives operate at 12X speed or greater. Such performance
puts significant strain on the motors and servo systems. Even 2X speed means
substantially higher demands of the electronics and power systems. Thus, you
may find that a drive will play audio CDs flawlessly but have trouble reading
data files. While there is probably little you can do to repair an electronic
or logical problem without schematics - which are almost certainly not going
to be available, there are some things to keep in mind which may lead to the
repair of problems like erratic or total failure of data readback.
The first test is to force the drive to the 1X (or some slower speed than its
maximum specifications) and see if that helps. Your drive may have come with
instructions/software to operate at a selected speed.
Data readout must be flawless. Uncorrectable errors which may not be noticed
for audio playback would result in corrupted files. Thus, anything that is
marginal may significantly impact performance. If it still has trouble with
data even at the 1X speed, something may be marginal or there may be a true
problem in the decoding logic or computer interface.
* Multi-X performance puts a much greater strain on motors and servo systems
so anything in the optics or servo alignment that is marginal - or even a
dirty lens - may result in problems which do not show up with audio CDs
played at the 1X speed. Thus, once a dirty lens is ruled out by cleaning
it, some fine tuning of the servo systems may be needed.
* Power requirements are substantially greater at the higher speeds - the
spindle motor and servos need to work harder and even the electronics
may require greater power. Therefore, weak batteries in laptop computers
or CDROM drives operated off of laptop power or an inadequate wall adapter
may result in erratic operation. If possible, try fresh batteries or a
different adapter before warming up the oscilloscope.

As with all equipment operated from a batteries, there are specific
requirements that must be met for reliable and safe operation.
Batteries must be of the proper type. Some devices will work on either
Alkaline or rechargeable NiCd types. However, since NiCds put out less voltage
than fresh alkalines, there may be a selector switch or the instruction manual
may state that NiCds should not be used. Batteries should be fresh - the
motors, servo systems, and electronics in a CD player or CDROM drive can be a
significant load when seeking or spinning up. A weak battery may cause it to
shutdown erratically or never be able to find the selected track. Do not mix
new and used cells. This can result in poor performance and may actually
result in damage to the cells where rechargeable (NiCd) types are involved.
Some CD players use a sealed lead-acid battery pack. For long life, these must
be recharged immediately after use. Leaving a lead-acid battery pack in a
discharged condition will significantly shorten its life. And these are not
cheap! A pack for a typical Sony CD player may cost more than $20.

As with all equipment operated from a wall adapter, there are specific
requirements that must be met for reliable and safe operation:
1. Voltage. The CD player or other device will specify the nominal input
voltage. This must be adhered to - you cannot connect a 3V CD player to
a 12 V adapter (or auto battery, for that matter). It will become toast.
However, not all wall adapters are created equal. Some are very poorly
regulated meaning that even though its label says something like '9V',
the actual output may be as much as double this (or more) with no load.
This may not be acceptable. The device may overheat or be damaged or
destroyed nearly instantly. Internal protection devices may blow (if you
are lucky!). It is safest to follow the manufacturer's recommendations
(though, admittedly, they may be pushing their own brand of adapter). My
rule of thumb is that if the unloaded output voltage is within about 25%
of the specified requirements, it is probably safe to use. However, when
connecting for the first time, be on the lookout for any strange behavior
(or strange odors!).
2. Current. The required current should be stated somewhere - either on the
device itself or in the instruction manual. If only power is specified
(i.e., 9 V, 4.5 W), then divide power in watts by voltage to get the
current rating in A. (1 A = 1000 mA). The adapter must be capable of
putting out at least this amount of current though a modestly higher
current rating should be no problem. Using an adapter with an inadequate
current rating may result in erratic behavior or overheating and failure
of the adapter.
3. Polarity. All the portable CD players and CDROM drives I know of operate
on DC. Thus polarity is critical. Get it backwards and at best nothing
will happen but nothing will work either. However, the equipment and/or
adapter may be damaged - permanently. Internal protection devices may
blow - if you are lucky.
4. Regulation. It is often impossible to determine whether the device expects
regulated power or whether a given AC adapter provides it without tests.
There are both types. Higher voltage AC adapters (say, 6 V or above) often
tend to be just rectifier/filter capacitor types. However, low voltage
adapters (e.g., 3 V) may have an IC regulator built in.
As noted in the section: "CD player is totally dead", it is easy to destroy a
portable device using an improper power adapter or a universal adapter that
is configured incorrectly:

These combine a stereo receiver and a single or dual cassette deck, and/or
a CD player or changer, and a pair of detachable speakers, into a single
unit. Most are fairly portable but larger boomboxes and compact stereos
may require a forklift to move any great distance.
While the individual subsystems - CD player for example - are usually
relatively self contained electrically except for a common power supply,
mechanically, everything tends to be jumbled together - even on units
that have an outward appearance of separate components. Both cassette
transports are usually driven from a single motor. Getting at the CD
player may require removal of both cassette decks, audio amplifier, and
power supply. Working on these is not fun. As usual, take careful notes
as you disassemble the unit and expect it to require some time just to get
to what you are after. Be especially careful when removing and replacing
the individual modules if printed flex cables are used for interconnections.
Refer to the relevant sections on cassette transports, loudspeakers, and
power supplies for problems with these units.
Since these do get abused - bumped, dropped, dunked, etc., bad connections,
and other damage is very common. See the sections: "Intermittent or erratic operation" as well as "Audio muting, noise, or distortion".

I have never heard of a component CD player being dropped or rained on.
However, this does happen to portables. While a service shop may not
even want to tackle such a unit, it is quite possible that damage is
minimal - even for a CD player.
With a CD player that has been dropped, unplug it from the AC line or
remove the batteries immediately. This will prevent further damage
should anything be shorting internally.
For one that has gotten wet, dry it immediately (you knew that!).
See the document: "Notes on the Troubleshooting and Repair of Audio Equipment
and other Miscellaneous Stuff" for more info on restoration of abused audio
equipment.